Method of preparing a filter medium for the filtration of molten aluminum or a molten aluminum alloy

ABSTRACT

A method of preparing a filter medium for the filtration of molten aluminum or a molten aluminum alloy comprising the steps of using more than two halogenide compounds, of which at least one halogenide is selected from the group consisting of compounds of sodium fluoride, potassium chloride, silver chloride, potassium fluoride and sodium nitrate, and of which the second halogenide is selected from the group of compounds having a melting point of more than 1000*C consisting of calcium fluoride, magnesium fluoride, aluminum fluoride, barium chloride, cryolite, and cerium fluoride and mixtures thereof; heating said compounds until they are molten and then mixing thoroughly with some agitation to obtain a homogeneous melt and subjecting the thus obtained melt to a molding step.

United States Patent 1 [111 3,907,962

Ogiso Sept. 23, 1975 METHOD OF PREPARING A FILTER MEDIUM FOR THE FILTRATION OF MOLTEN ALUMINUM OR A MOLTEN ALUMINUM ALLOY Koichi Ogiso, 25 Tashiden, Daito, Osaka-fu, Japan Filed: Feb. 17, 1972 AppL No.: 227,303

Related US. Application Data Continuation-impart of Ser. No. 842,294, July 16, 1969, abandoned.

Inventor:

References Cited UNITED STATES PATENTS 8/1970 McDonald et a1. 210/496 Primary ExaminerGerald A. Dost Attorney, Agent, or Firm-George B. Oujevolk [57] v ABSTRACT A method of preparing a filter medium for the filtration of molten aluminum or a molten aluminum alloy comprising the steps of using more than two halogenide compounds, of which at least one halogenide is selected from the group consisting of compounds of sodium fluoride, potassium chloride, silver chloride, potassium fluoride and sodium nitrate, and of which the second halogenide is selected from the group of compounds having a melting point of more than l000C consisting of calcium fluoride, magnesium fluoride, aluminum fluoride, barium chloride, cryolite, and cerium fluoride and mixtures thereof; heating said compounds until they are molten and then mixing thoroughly with some agitation to obtain a homogeneous melt and subjecting the thus obtained melt to a molding step.

11 Claims, 6 Drawing Figures US Patent Sept. 23,1975 Sheet 1 of2 3,907,962

METHOD OF PREPARING A FILTER MEDIUM FOR THE FILTRATION OF MOLT'EN ALUMINUM OR A MOL'IEN ALUMINUM ALLOY OBACKGROUND on THE INVENTION.

This application is a continuation-in-part of US. patent application Ser. No. 842,294 filed on July 16, 1969 and now abandoned.

This invention relates to the purification procedure of molten metal. Heretofore, for this purpose gas blowing methods, treatment with various fluxes, and methods of filtration by glass wool or ceramics have been widely used. However, these treatments emphasize mechanical, but not physical chemistry characteristics.

This invention relates particularly to a novel treatment of molten metal based on physical chemical properties, especially the interfacial phenomenon and also provides for a filtering medium and a filtering method. Molten metal contains oxides as impurities, and nonmetallic inclusions produced by partial separation arising from furnace walls and foundary tools, as well as gaseous impurities such as hydrogen diffused through the oxide film, and, it is difficult to obtain normal molten metal without eliminating the impurities. The oxides most often included in the impurities are very stable ionic crystals and always porous. Therefore, the polar gases diffused into molten metal are absorbed on the surface of the oxide and diffuse through the porous inner part. Abnormal molten metal has numerous interfaces of molten metal and impurities and also interfaces of oxides and gases in impurities.

THE PRIOR ART It is well known that to obtain a high purity aluminum metal by the method wherein molten aluminum is filtered by using a filter medium which is made of inert and fire-resistant materials, such as aluminum oxide, andcarbon element substances, and chamotte substance, and encased in a suitable vessel, on filtering, the impure materials contained in the aluminum metal are usually filtered off, but in this case, the interfacial tension between the molten aluminum and the filter medium is so large that the following two defects are to be noted owing to the pore size of the filtrate path which is controlled by regulating the size of the filter medium substance used. The first defect isthat it takes a long time to, filter through the path pores which are a little smaller than those usually used, and the second defect is that the conventional filtering processes are unsatis factory if the path pores are a little larger than the conventional ones, because the impurities of tiny particles readily pass through thefilter medium pore paths. And thus, these defects prevent the proper industrial use of conventional filters.

Considering the purifying effect on the industrial process, the filter medium should be made of substances which will have as small an interfacial tension as possible between the molten metal and filter medium. Now, according to the present invention, there is provided a filter medium which fulfils this purpose. In the production process of the medium according to the present invention, some fusible materials such as sodium chloride, potassiumchloride, potassium fluoride, silver fluoride, and sodium nitrate may be used.

On the other hand, it is a noticeable fact that there are some men skilled in-the art who support theprobability of the ready removal of impurities, i.e., the oxide compounds which are contained in aluminum metal, from the molten aluminum when the sum of the interfacial tension of alumina/flux system and that of aluminum/flux system has become smaller than the interfacial tension of alumina/aluminum system.

. In the technical field it is well knwon that there exists the phonomenon of gas absorption of the oxide substances in the molten metal phase, and this phonomenon is understood as a physical one in which non-polar gas components are absorbed into the substances of ion crystal structure, wherein the absorption energy is generally rather small. Therefore it is possible to suppose that by giving some other type of larger energy onto the interfacial system, and by breaking up the boundary surface or the equilibrium state between surfaces, some absorption relating to the gas component would be generated by some macro-physical decomposing reaction relating to the gas absorbed material.

Regarding conventional processes for removing impurities, such as oxide materials which are dispersed in the molten metal, it is easy to filter off large sized impurity particles by catching them on the filter medium layer, but it is not so easy to filter off the impurities of small sized particles, because small particles of impurities pass through the pores in the medium layer without hindrances. This is the bottleneck of the filteration process. To solve such a problem of the conventional filtering process it is necessary to use some filter medium which has superior physical and chemical properties to absorb the impurities of the ultra-tiny particles. And thus, it is preferable to use chemically and physically active fire-resistant substances than to use inert fireresistant substances in order to obtain a better absorbing efficiency from the filter medium, and consequently, a better filtering effect.

SUMMARY OF THE INVENTION This invention is achieved by the knowledge of the characteristics of interfaces in molten metal and is based on a filtering material which absorbs or combines the impurities together with the reduction of interfacial energy produced in the interfacial phase and the decrease of interfacial force, and also is based on a method to provide impact energy by means of the destruction of interfaces between oxides and gases by the reduction of interfacial energy. A filtering bay is provided for the place to carry out this invention.

According to the present invention, materials which contain sodium chloride, potassium chloride, or potassiumfluoride may be used as a suitable filter medium. Thus, the interfacial surface tension which is produced by both filter medium and the molten metal to be filtered becomes small. The filter medium materials of the present invention which contain ion crystals of sodium compounds, calcium compounds, or lithium compounds are effective to absorb hydrogen gas which is dispersed in the substrate molten aluminum metal. In this case it may naturally be considered that the metal element of the compound compositions of the substance medium is converted, by touching the hydrogen gas generated in the molten metal phase, into some compounds such as sodium hydride, calcium hydride, and lithium hydride. During the metal filtering process the quantity of the active compound material in the filter medium has considerable influence on the impurity removal properties of the filter medium.

In one aspect of the present invention the chemically related materials with a melting point of more than lOOC, such as metal halides, e.g., barium fluoride (BaF calcium fluoride (CaF cerium fluoride car cryolite Na AlF magnesium fluoride (MgF and aluminum fluoride (AlF which have desirable properties of being able to resist against wearing out, or being extinguished off in the course of use, and to be active to the impurities in the molten aluminum metal, should also be contained in the filter medium as constructional ingredients. it is further preferable that the filter medium have some compound material such as carbon compound, nitrogen compound, boron compound, or silicon compound, in order to acquire the permanency of the filter medium for a long lasting use. A filter medium which is cast by conventional molding methods and contains usual materials or usual materials with some binding material is apt to disintegrate and be extinguished during use. This means inferior filtering efficiency. On the other hand, according to the present invention, the material of the filter medium comprises of at least two components selected respectively from the following two groups, i.e., one is a group which contains sodium chloride, potassium chloride, potassium iodide, silver chloride, sodium nitrate, and another is a group which contains compounds with a melting point of more than l0OOC, so as to get a filter medium which is satisfactory after being used many times without failure, and able to resist melting or wearing away, and having a superior filtering effect.

Another aspect of the present invention is that the filter medium material may contain some halide compound of either sodium metal, calcium metal, or lithium metal which has superior properites to absorb hydrogen. According to the present invention it is preferable to use one or more compounds of carbon, nitrogen, boron, and silicon-containing compounds, and the materials which contain more than two kinds of metal halides as manufacturing raw materials for the filter medium preparation.

One of the main objects of the present invention consists in providing a filter medium of superior quality wherein the constructional material of the medium comprises of at least more than two kinds of the aforesaid metal halides and any one or more than one compounds selected from the group of carbon, nitrogen, boron, and silicon containing compounds, and the manufacturing procedure of the filter medium is effected by melting the above mentioned raw constructional material at a high temperature, blending and mixing the molten mixture thoroughly with mild agitation and pouring the molding material thus obtained into a mold at atmospheric pressure to obtain a desired filter medium. The molding mechanism resembles the casting mechanism used for metal alloy casting.

Yet another object of the present invention is to provide a filter medium of superior and improved permanency to resist against wear after multiple use. To attain this purpose the filter medium may be manufactured by using either of the following three constructional molding raw materials, i.e., the foregoing metal halide substances; a mixture of any one or more than one compounds selected from the group consisting of carbon, nitrogen, boron and silicon containing compounds and the foregoing halides; or a mixture of any one or more of the group of materials consisting of carbon, nitrogen, boron and silicon containing compounds. These halides and the inert and fire-resistant. substances, such as ceramics, glass wool, carbon fiber, and occasionally, some suitable metal powder or metal particles may be added to the above mentioned cases.

The composition of the molding materials for the filter medium of the present invention are as follows:

Composition Example 1 Ingredients Content (3%) based on the mixture weight Calicum fluoride 10- 40 Borax 5 l5 Magnesium fluoride 30 Man-made cryolite 5 l5 Lithium carbonate 5 15 Sodium chloride 5 [5 Composition Example 2 Barium fluoride 30 60 Potasium chloride 10 30 Sodium chloride 5 20 Potassium fluoride 5 20 Lithium chloride 5 2O Composition Example 3 Potassium fluoride 20 40 Sodium chloride 20 40 Alumina l0 30 Composition Example 4 Calcium fluoride 30 50 Lithium chloride 10 20 Sodium nitrate l0 20 Titanium metal powder 5 10 The raw materials of each example are heated to about lO00Cl400C, i.e., to their melting points with thorough stirring after the substances become liquid. Each composition thus obtained is cast into a mold of graphite with a rapid cooling treatment so that such disadvantageous effects, such as segregation phenomenon, or some phase separation phenomenon between the ingredients does not take place.

The composition of Example 1 comprises metal halides and a carbon compound. The composition of Example 2 comprises metal halides only. The composition of Example 3 comprises metal halides and a ceramic substance. The composition of Example 4 comprises metal halides, a nitrogen compound and metal powder.

The filter medium made of each composition in Examples l to 4 has a striking permanency and endures against wearing out in severe circumstances of high temperatures of the molten aluminum phase.

Each filter medium made of the composition in Examples 1, 3 and 4 has a specific gravity of value range of from about 2.4 to 2.6, and these mediums can be conveniently used for the metal filtering process by putting them in a piled mass form between two porous filter walls in the molten aluminum metal. On the other hand, the filter medium made of the mixture of Example 2 has a specific gravity of about 3.0 to 3.2, and can be conveniently used for the process by putting the medium in a piled mass form on a porous filter plate wall at any place in the molten aluminum metal, or on a same 'kind of plate wall positioned at the bottom side of the molten metal liquid to be filtered.

The characteristics of the filtering material of this invention are its high ionic bonding force, the increasing of the ionic bonding force in the molten metal, its entire non-destructiveness by shocks, and to exclude binders such as ethane hexachloride to melt mold the composition such as metal halogenides, and to obtain stable crystals for long hours and structurally homogeneous and isotropic to mold in high-cooling of the mol- The filtering material of this invention continuously produces a very stable viscous molten salt film on the surface of the filtering material.

The film reduces the interfacial energy between molten metal and impurities and decreases the interfacial tension and also absorbs the impurities-by the wetting. This molten salt film is slightly ionized by the decrease of crystallinty and continuously generates a gaseous product at a very slow speed.

This gaseous product eliminates gaseous impurities such as hydrogen.

The gaseous product and free ionized atoms generate the gaseous atmosphere or the thin molten compound film with metal on the surface of molten metal, and prevent the contact of the atmosphere with the molten metal and avoid oxidation of the molten metal for long hours and simultaneously destroy the oxide film and keep the metal in a normal state by reduction of the diffusion into the oxide layer.

The filtering material of this invention is odorless and smokeless and is effective for the purification procedure for more than 100 work hours with one unit, and by using it, stable molten metal is obtained.

The filtering material containing powder or a mass of fire and heat resistant materials, metals and various fibers such as glass fiber or carbon fiber are used to mold low melting point compositions as shown in the Composition Table and to mold materials of high specific gravities than molten metal.

This procedure is also used to produce layers of metal halogenide salts on the surface of the mass of fire and heat resistant materials and metals.

In filtering materials including the above mentioned substances, the surface crystals of the molded materials are activated gradually to ionize ions or liberate gases; there is a tendency of the said filtering material to shrink in suface area. However, the existence of heat and fire resistant substances which are exposed at their existing positions in said filtering material will protect the surface area from being reduced and the filtering passes from being expanded, so that the impurities can I eration of an oxide film of molten metal to intercept the contact of the surface of the molten metal with the atmosphere, which is formed by the contact of gaseous material with the surface of the molten metal. Furthermore, another object of this invention is to decompose the oxide film by the contact of ions with molten metal preventing the diffusion into the oxide layer. These procedures keep the molten metal normal for long hours. Y

' As the oxide and gasrirnpurities are absorbed with low energy, the interface of oxide and gas 'is easily decomposed by the impact with piled filtering material.

As for the place of piling the filtering material, it is recommended to provide a filtering bay in the vessel. The structure of the filtering bay is composed of a vessel which piles and accomodates the filtering material of this invention and has porous holes through which the molten metal can be passed, a buffer plate and a cylinder form a sieve.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a vertical section of Example 5 of this invention;

FIG. 2 shows a vertical section of Example 6 of this invention;

FIG. 3 shows a vertical section of Example 7 of this invention;

FIG. 4 shows a vertical section of Example 8 of this invention;

FIG. 5 shows a vertical section of Example 9 of this invention; and,

FIG. 6 shows a graph useful in this invention.

DETAILED DESCRIPTION Example 5 I In a melting vessel 11 as shown in FIG. 1, a flanged open vessel 12 which is made of graphite or ceramic is floated when the material has a heavier specific gravity than the molten metal, by a float ring or an air vessel 14 of asbestos and by a floating force when the material has a lighter specific gravity than the one of molten metal.

The part 13 designates a porous bottom wall of vessel 12 and a porous cover plate 15 closes the vessel which contains filtering material A piled upon the bottom plate. Molten metal passes theporous bottom plate and is purified by the filtering material A and pumped up from the flanged open mouth of the vessel 12 passing the porous cover plate 15.

The vessel 12 can be fixed with a support or a supporting tool.

Example 6 In a melting vessel 21 as shown in FIG. 2, a buffer plate 22 which is composed of graphite, ceramics or metal, a porous bottom plate 23 and an upper plate 24 are installed, and the filtering material A is piled on the bottom plate and closed by the upper plate 24. Molten metal passes the porous bottom plate 23 combined with the buffer plate and is purified by the filtering material and then pumped up passing the porous upper plate'24.

Example 7 In the melting vessel 31 shown in FIG. 3, a vessel 32 made of ceramic and a pair of porous walls 33 connected with the vessel 32 below andseparated a definite distance form a room having a definite volume in which the filtering material A is packed. The side wall 33 can be a box such as a cylinder. Molten metal passes the porous front side wall and is purified by the filtering material A and then pumped up passing the porous back side wall.

Example 8 To the melting vessel 41 shown in FIG. 4, a takeout mouth 42, a porous wall 43 are installed. The filtering material A is packed in the room formed by the porous walls 43 placed at a definite distance. Molten metal passes the porous wall below and is purified bythe packed filtering material A and pumped up by the takeout mouth 42 passing the upper porous wall.

Example 9 its purifying effect upon the molten metals.

Next, the oxidation products included in the molten aluminum purified with the filtering material of this invention and one not purified are quantitatively analyzed. About 500 kg of aluminum 18 wire waste is melted gradually for 110 hours in a graphite crucible and test pieces are sampled in every 10 hours by a In a sealed low pressure molding metal vessel 51 as graphite form. An untreated sample means one unpurishown in FIG. 5, the lower part of a low pressure moldfied in molten state. ing tube 52 is packed with the filtering material A and Analysis Table I installed w1th a porous plate 53, and a porous cover 54 upon the piled filtering material A. Sample A] o (1%) The porous bottom plate and cover plate can be in- Untreated SamPe P ifi d sample stalled in the upper part of the tube, and also an at- (39) tached structure can be placed underneath of the bot- Immediately after 00010 00005 tom part. Molten metal passes the porous bottom plate After 10 hours 0.0015 0.0008 and is purified by the filtering material A and injected g8- 388?; under pressure from the tube to cast 55 passing the po- I'OUS cover plate 54. Untreated Sample: Test made without use of the filtering material Aluminum p Aluminum Casting Stark Purified Sample: Test made with use of the filtering material dard) is molded using a low pressure casting tube made The filtering medium used 100 hours in Analysis of ra bite in to Fl er n i i of t he cylintiffh e zid if 0.5 5g 2:31:51, Table 1 cruihed l mused The of 10 to 20 Seconds About 2000 pieces of ingot were effect of filtering materlal to the alummum alloy are tested and checked for the harmful effect of the filtercast during 80 hours of continuous operatlon and checked, resulting in one defective sample in processmg matenal to metal lug. Analysis Table 2 In this test, blowing phenomenon with flux, mixing of the oxidation product and creases by the mixing of Sample A1,0,(%) oxide film are hardly observed. tfgf ii g Instead of compressed air inert gases can also be used Immediately after 0.00l6 0.0007

The effect obtained by the parification procedure After hours (mom based on the above mentioned examples are described with regard to molten aluminum and a molten alumium a|loy Analysls Table 3 The filtering materials used in the aforementioned examples have the following compositions: kr i i wire Time Valu Si(%) Fe(%) Cu(%) Mg(%) Ingredient n nt based on the Before the 0.59 0.62 0.13 0.77

mlxwre welght ii rffii d i t ely after Calcium fluoride l0 4O 40 the treatment 0.86 0.6l 0. l4 0.34 r x 5 15 After 30 hours 0.57 0.62 0.13 0.31 mf r t ri ziiie c i 'y l i t (2) Material: Al-Si l 1% Alloy 1 32;535:5222: Time A3352 Si(%) Fe(%) Cu(%) Mg(%) Ca(%) Na(%) Sodium chloride 5 2O Before the A mixture of the above mentioned composition is treatment 1098 0002 heated to about l200C with agitation to melt homoge- After the neously and then this molten mixture is fed into a 50 mama 0003 graphite mold. The micro-structure of the molded goods obtained are homogeneous and the phenomenon of segregation is not apparent. These cast materials are Al-Si 11% alloy of material (2) in Analysis Table 3 active in molten aluminum and impurities, and absorb is purified twice according to the purification method and remove the impurities by their wetting properties. of Example 5 and die-cast and 2290 finished pieces are Furthermore, a trace amount of fluorine and chlorine used for a cover case for an automobile. Only one piece gases continuously produced eliminates mechanically of defective product was observed comparing the the gases such as hydrogen contained in molten metal O.4-l% defect ratio used in conventional gas blowing and decomposed the oxide film generated on the suror fluxing. The method by this invention reduces the face of the molten metal producing a soft and thin comdefect ratio to less than 0.0004%. When the return mapound and prevent the oxidation of molten aluminum terial coated with oil-cake or alumite treated is used, together with the prevention of the diffusion of gases the molten metal is recommended to be purified acsuch as hydrogen. Even in the case where said filtering cording to this invention to obtain normal molten metal material consists of a molding of a kind of metallic salt, with less slag and to increase product yield. it shows similar behaviors in molten metals and exerts Furthermore, 63S billets were cast by a conventional gas blowing method using the purification procedure described in Example 6 of this invention during about 2 months. The results are shown in Graph 1. Cast pieces are tested by an ultrasonic testing method with the conditions of 0.6 of pulse, 5.0 of sensitivity, and the results are ranked Y.B.C.D. for pass, X for fail.

In the graph, the upper hatched line is by the gas blowing method, and the lower gray line is by the Example 6 of this invention.

When plates and wires such as once-drawn wires are continuously casted, the yield is improved to 100 percent/Y-rank and it is clear that the improvement and stabilization of the quality are attributed to this invention.

Small amounts of the samples of the filter material collected in every hours for the samples for Analysis Tables 1 and 2 are also qualitatively analyzed by spectrophotometry and quantitatively by chemical method and showed in Analysis Tables 4 and 5.

Analysis Table 4 melt and subjecting the thus obtained melt to a molding step.

2. The method as claimed in claim I, wherein the filter medium is admixed with at least one halogenide compound having a metal ingredient selected from the group consisting of calcium ion, sodium ion, and lithium ion which have the property to produce hydrogen compounds.

3. The method as claimed in claim 1, wherein there is added to the filter medium one or more than one compounds selected from the group consisting of carbon, nitrogen, boron, and silicon containing compounds.

4. The method as claimed in claim 2, wherein there is added to the filter medium one or more than one compounds selected from the group consistings of car- Filter Material (about main components only) time before After treatment (hrs.)

treatment I Component i0 Mg 2795 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 +4 Na 3302 +3 +2 +3 +3 +2 +3 +3 +3 +3 +3 +3 Ag 3382 +2 +2 +2 +2 +2 +2 +2 +2 +2 +2 +2 Ca 3933 +5 +5 +5 +5 +5 +5 +5 +5 +5 +5 +5 Al 3961 +3 +3 +4 +4 +3 +5 +5 +5 +5 +5 +5 Mn 4034 +2 +2 +2 +2 +2 +2 +2 +2 +2 +2 +2 Analysis Table 5 bon, nitrogen, boron, and silicon containing compounds. Quantitative Chemical Analysis (the methods 5. The method as claimed in claim 1, wherein there dflemmm" abbwvaied) 35 is added to thefilter medium at least one fire-resistant Hour Ca (36) Mg (11) Ba ($1) material selected from the group consisting of ceram- Before treatment 10.34 14.41 12.17 ic's, glass wool, and carbon fiber. 22:: g 2:2; {gig H12: 6, The method as claimed in claim 2, wherein there After 100 hours .68 17.06 10.23 is added to the filter medium at least one fire-resistant 40 material selected from the group consisting of ceram- From Analysis Tables 4 and 5 and X-ray diffraction, it is clear that the filter materials have no structural change. The weight of the filter materials are determined every hour. The weight loss is 0.3 to 0.5% per hour until 60 hours and then the weightless decreases gradually and atl00 hours, no weight loss is observed. The weight loss is caused by the release of the gas product.

I claim:

l. A method of preparing a filter medium for the filtration of molten aluminum or a molten aluminum alloy comprising the steps of using more than two halogenide compounds, of which at least one halogenide is selected from the group consisting of compounds of sodium fluoride, potassium chloride, silver chloride,.potassium fluoride and sodium nitrate, and of which the second halogenide is selected from the group of compounds having a melting point of more than l000C consisting of calcium fluoride, magnesium fluoride, aluminum fluoride, barium chloride, cryolite, and ce- ,rium fluoride and mixtures thereof; heating said compounds until they are molten and then mixing thoroughly with some agitation to obtain a homogeneous ics, glass wool, and carbon fiber.

7. The method as claimed in claim 3, wherein-there is added tothe filter medium at least one fire-resistant material selected from the group consisting of ceramics, glass wool, and carbon fiber.

8. The method as claimed in claim 1, including the step of admixing metal powder, or metal lump, of which the melting point is higher than the aluminum melting point before the melting step.

9; The method as claimed in claim 2, including the step of admixing metal powder, or metal lump, of which the melting point is higherthan the aluminum melting point before the melting step.

10. The method as claimed in claim 3, including the step of admixing metal powder, or meal lump, of which the melting point is higher than the aluminum melting point before the melting step.

11. The method as claimed in claim 5, including the step of admixing the metal powder, or metal lump, of which the melting point is higher than the aluminum melting point before the melting step. 

1. A METHOD OF PREPARING A FILTERED MEDIUM FOR THE FILTATION OF MOLTEN ALUMINUM OR A MOLTEN ALUMINUM ALLOY COMPRISING THE STEPS OF USING MORE THAN TWO HALOGENIDE COMPOUNDS, OF WHICH AT LEAST ONE HALOGENIDE IS SELECTED FROM THE GROUP CONSISTING OF COMPOUNDS OF SODIUM FLUORIDE, POTASSIUM CHLORIDE, SILVER CHLORIDE, POTASSIUM FLUORIDE AND SODIUM NITRITE, AND OF WHICH THE SECOND HALOGENIDE IS SELECTED FROM THE GROUP OF COMPOUNDS HAVING A MELTING POINT OF MORE THAN 1000*C CONSISTING OF CALCIUM FLUORIDE, MAGNESIUM FLUORIDE, ALUMINUM FLUORIDE, BARIUM CHLORIDE, CRYOLITE, AND CERIUM FLUORIDE AND MIXTURES THEREOF, HEATING SAID COMPOUNDS UNTIL THEY ARE MOLTEN AND THEN MIXING THOROUGHLY WITH SOME AGIATION TO OBTAIN A HOMOGENEOUS MELT AND SUBJECTING THE THUS OBTAINED MELT TO A MOLDINP STEP.
 2. The method as claimed in claim 1, wherein the filter medium is admixed with at least one halogenide compound having a metal ingredient selected from the group consisting of calcium ion, sodium ion, and lithium ion which have the property to produce hydrogen compounds.
 3. The method as claimed in claim 1, wherein there is added to the filter medium one or more than one compounds selected from the group consisting of carbon, nitrogen, boron, and silicon containing compounds.
 4. The method as claimed in claim 2, wherein there is added to the filter medium one or more than one compounds selected from the group consistings of carbon, nitrogen, boron, and silicon containing compounds.
 5. The method as claimed in claim 1, wherein there is added to the filter medium at least one fire-resistant material selected from the group consisting of ceramics, glass wool, and carbon fiber.
 6. The method as claimed in claim 2, wherein there is added to the filter medium at least one fire-resistant material selected from the group consisting of ceramics, glass wool, and carbon fiber.
 7. The meThod as claimed in claim 3, wherein there is added to the filter medium at least one fire-resistant material selected from the group consisting of ceramics, glass wool, and carbon fiber.
 8. The method as claimed in claim 1, including the step of admixing metal powder, or metal lump, of which the melting point is higher than the aluminum melting point before the melting step.
 9. The method as claimed in claim 2, including the step of admixing metal powder, or metal lump, of which the melting point is higher than the aluminum melting point before the melting step.
 10. The method as claimed in claim 3, including the step of admixing metal powder, or meal lump, of which the melting point is higher than the aluminum melting point before the melting step.
 11. The method as claimed in claim 5, including the step of admixing the metal powder, or metal lump, of which the melting point is higher than the aluminum melting point before the melting step. 